The present invention is directed, in general, to wireless communications and, more specifically, to addressing multipath fading in wireless communications through transmitter diversity with efficient encoding.
In wireless channels, where signals can arrive at different times following different paths, the received signals can experience large amplitude and phase variations due to the interference of the different paths. This phenomena is referred to as multipath fading, and the effect is a critical design parameter for consideration in receiver design. For terrestrial mobile telephony, multipath fading may dictate the entire system capacity and throughput rate.
Wireless signals transmitted from or to a mobile station may be reflected from the terrain, fixed or mobile objects in the propagation path such as buildings or vehicles, or from a discontinuity in the atmosphere. The energy of the reflected wireless signal is not significantly absorbed and/or attenuated, creating a plurality of different propagation paths for the wireless signals between the transmitter and receiver, referred to as multipath propagation, which allows the wireless signals to xe2x80x9cbendxe2x80x9d around corners and propagate beyond terrain features and objects obstructing the line-of-sight between the base and mobile stations.
Three problems associated with multipath propagation for mobile stations include (1) the delay spread of the received signal, (2) the Rayleigh fading in received signal strength caused by varying phase shifts between different paths, and (3) the varying frequency modulation due to the Doppler shift between various propagation paths. The fact that propagation paths for reflected signals are longer than the direct propagation path from the transmitter to the receiver (e.g., from the base station to the mobile station) gives rise to signal delays and, because various paths lead to slightly different arrival times, the received signal xe2x80x9cspreads.xe2x80x9d Rayleigh fading results from differences between the phase and amplitude of the reflected wireless signals relative to the phase of a directly propagating signal, attenuating the signal strength at the receiving end (e.g., reception of two signals propagated along two different paths and arriving with a phase difference of 180 degrees results in cancellation in the receiver). Doppler shift is caused by the movement of the mobile stationxe2x80x94or a vehicle or other reflecting objectxe2x80x94in relation to the base station, such that the mean frequencies of both the received reflected signal and of the directly propagated signal deviate from the mean frequency of the transmitted signal by a different amount and in a different direction.
In general, multipath fading causes wide variations in received signal amplitudes, and much effort has been expended in attempting to mitigate the impact of multipath fading. One suggestion for minimizing the effects of multipath fading, set forth in the Electronics Industry Association Telecommunications Industry Association (EIA/TIA) proposed standard IS-2000, employs space-time spreading (STS), in which identically coded data frames are transmitted on each of two (preferably orthogonal) channels utilizing physically displaced antennas. In orthogonal transmit diversity (OTD), the identically coded data streams are demultiplexed onto the two antennas.
The underlying rationale for space-time spreading is that the two channels are unlikely to fade at the same time. However, the transmission of identical data on both channels is unnecessarily wasteful, and demultiplexing data from a single code rate onto two different antennas provides less than optimal performance. There is therefore a need in the art for improving the efficiency of space-time spreading transmit diversity in overcoming multipath fading.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless communications system, a mechanism for employing turbo coding together with space-time spreading (STS) transmission to improve transmission error performance and efficiency in addressing multipath fading. Different subsets of parity data are transmitted via each antenna/channel, with either subset alone sufficient for error recovery. In general, however, both channels will be received with some fidelity, and the parity data from the two channels is combined and decoded as a lower code rate transmission. Systematic data may be transmitted on both channels, with or without interleaving on an alternate channel. Time diversity may also be introduced by transmitting only part of the parity data within the two parity data subsets, then transmitting the remainder if the receiver could not produce the correct decoding from the original transmission.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.